my recommendations are EvoIII 16G from Forced Performance or Blouch. An APS or Turboxs Top mount IC, 650cc injectors, and a walboro fuel pump. Thats just for the turbo with it's direct supporting mods, it appears your at a stage 2 setup. Thats going to net you 300+ whp with a good tune (which will run 300-400). With an 03 wrx you will not even come close to an 800hp mustang. You would need a complete built race engine. As dumdum said earlier, this is all aside from upgrading your tranny. If you go with the turbo I suggested you'll still need to baby the tranny. Either way you're looking at spending a lot of money.

The best idea yet, take your 800hp mustang friend to an autocross track, or even a circuit with a good amount of corners and kiss him goodbye. You can laugh as you drive with one hand looking in your rearview of him spinning out all over the place. Many people don't understand these cars strong points are not in a straight line.